Mobile gas liquefaction platform



Dec. 15, 1964 P. c. KEITH ETAL 3,161,492

MOBILE GAS LIQUEF'ACTION PLATFORM Filed Aug. 25, 1961 2 Sheets-Sheet l INVEN'FORS PERCIVAL C KEITH ANDREWS D. KORIN GEORGE E FARKAS f, ATTORNEY Dec. 15, 1964 P. c. KEITH ETAL 3,161,492

MOBILE GAS LIQUEFACTION PLATFORM Filed Aug. 25, 1961 2 Sheets-Sheet 2 & g a .E .Q o g 3 m: m 3 Z 0') m 2- a x Q Hum E: i' lll E 3 w cu D 2 T w g a; H 9 L: l- 3 m 0 v E L 7/ g g d r n I 4 V o INVENTOR. N (:1 PERCIVAL c. lEllTH o y ANDREW 0. K R N (D B GEORGE q. gm 1 u. o;

3,161,492 MQBILE GAS LIQUEFACTEQN PLATFIPRM Percival C. Keith, Peapaclr, and Andrew D. Karin,

land, N1, and George B. Farhas, Jackson Heights,

N.Y., assignors to Hydrocarbon Research, Inc, New

York, N.Y., a corporation of New .Iersey Filed Aug. 25, 1961, er. No. 134,668 Claims. (Cl. 62-4tt) where the gas is available at remote non-industrial areas,-

it has proved to be a formidable technical problem to construct a commercial plant at the well-head. It will be appreciated that a cryogenic plant of suitable proportions requires elaborate precision machinery such as boilers, prime movers, turbines, compressors, heat exchangers, and delicate instrumentation to assure continuous economical operation. Insulation of the equipment against temperatures of .258 F. in itself is a major item of cost. In some cases, it is estimated that the cost of erection and start-up of atypical plant is as great as or greater than the cost of all the equipment.

This invention, while not necessarily so limited, is primarily adapted to the constructionand operation of a plant for the liquefaction of natural gas which plant would be shop assembled at a suitable industrial area on a mobile platform that can be transported to the well head and placed in operation with the minimum of technical assistance. It is so designed as to permit direct loading'of the liquid into transport means such as a ship or tanker.

More specifically, the invention contemplates the preassembly of a simplified cascade liquefaction plant on a marine platform which can be towed to a marine site near appear from the following description of a preferred form of embodiment thereof, taken in conjunction with the drawings attached, and in which:

FIG. 1 is a schematic View of a model of a marine platform containing the units of a gas liquefaction plant.

FIG. 2 is a schematic flow diagram of the principal units of a gas liquefaction plant.

As shown in FIG. 1, the platform is in the nature United States Patent 0 of a floating seadr-ome or marine platform which is may be shop assembled.

The preferred type of marine platform is provided with a plurality of caissons 15 which may be of a telescoping nature such as shown in the United States Patent 2,775,869 to Pointer and others, and by suitable means not shown, the adjustable parts'16 will be projected angina Patented Dec. 15, 1964 downward from the platform 10 until they engage the marine floor generally illustrated at 17. The entire platform may then be raised out of the water, the water level being indicated at 18.

As schematically illustrated, the platform will have a utility section not only for the service and maintenance of the platform and its operators but for the power needs of the liquefaction plant. Without intending to be restricted thereto, the platform is indicated as having one or more steam boilers 2t whichwill be gas fired and adapted to produce suitable steam'as for driving turbines 21 which in turn operate generators 22 for the necessary electric energy. Such turbines are usually run condensing with a condenser shown at 23. The utility section V will also have switch gear shown at 24 and standby or auxiliary turbo-generators shown at 25. In addition, the platform will have forced draft fans 26 and, of course, shops, storage rooms, crews quarters, etc. not herein specifically identified.

The major equipment on the platform 10 is the gas liquefaction plant. As shown in FIG. 2, it is based on a stagewise temperature reduction of natural gas to liquefact-ion temperatures, which at atmospheric pressure are about 258 F. The use of cascade refrigeration, using a plurality of refrigerants as hereinafter described, permits most economical cooling of the gas and the raising of the energy level so that the heat can be dissipated to water. By the use of the platform, the ocean cooling water permits a considerable saving of compression horsepower as will be discussed hereinafter.

The liquefaction cycle schematically shown in FIG. 2 provides for the incoming gas at 48 to be washed in caustic wash tank 42 and then cooled in exchanger 44 which is preferably cooled by high temperature level propane as hereinafter described. The condensed water is separated out in tank 46 and the partially dried gas is then passed through drier 48 in which the rest of the water is removed.

The dry gas discharging at-50 is then cooled in a series of heat exchangers such as 52, 54, 55, and 55 with heat removal sources including the overhead fuel gas stream 58, usually one or more levels of propane In the nitrogen stripper the pressure is such that a fuel gas is removed overhead at 58 with most of the nitrogen and some methane so that a stabilized liquid methane can be removed from the bottoms at '76. A reboiler 78 will be used ordinarily on this stripper, the

heat being supplied by any suitable available stream.

The stabilized liquid drawn ofi"v at '76 is then subcooied at 8d and at 82 with the final stage preferably.

accomplished, by nitrogen refrigeration down to a temperature of 258 F. or lower for storage purposes.

The cascade refrigeration system includes interconnected propane, ethane and methane circuits as well as a nitrogen circuit. While all of these circuits are not shown, they are similar to the propane circuit which is illustrated. In this case the liquid propane from accumulator liltl drawn off by line 101 is expanded through valves Th2 and Ella in passing to flash drum 103. Some of the liquid propane passes through heat exchanger 44 where the heat of the feed gasis sufficient to boil the liquid. The vapor from'heat exchanger 44- 'together with Q) the balance of the liquid from drum 100 then enters fiash drum 103. The gas from flash drum 103 then passes through line 104 back to the third stage compressor 106.

The liquid drawn off from flash drum 103 through line 108 is similarly expanded through valves 109 and 109a with part of the liquid passing through a bank of heat exchangers including heat exchanger 54. The liquid gas mixture in a similar manner to the liquid-gas feed to drum 103 then passes into the second flash drum 110. This flash drum is at the suction pressure of the second stage compressor 112 to which the gas is fed through line 114.

Liquid withdrawn from drum 1th through line 116 is expanded through valve 118 and used to cool a series of exchangers including the heat exchanger 55 above mentioned and is then returned by line 119 through surge drum 120 and line 121 to the first stage compressor 122.

The first stage compressor 122 is interconnected at 123 with the inlet 114 to the second stage compressor H2 and the second stage compressor in turn is connected through line 125 with the inlet 104 to the third stage compressor 106. The high pressure propane removed through line 127 is then water cooled in heat exchanger 123 for condensation and accumulation in the accumulator 100.

Preferably, a refrigerant such as propane, will be used etficiently in two or more stages to permit incremental cooling in the natural gas line. In a similar manner, an ethane circuit and a methane circuit will be used for other natural gas cooling heat exchangers as shown at 56, 66, 6S and St The respective refrigerants will also be used in intercooling as for example, using propane to remove the heat of compression in the ethane circuit. Generally the number and arrangement of refrigerant cooled heat exchangers will be based on well known thermodynamic laws and will permit a suflicient difference to minimize the heat exchange surface.

The nitrogen cycle is required only at the single level of 265 F. for the heat exchanger 82. However, for economy the nitrogen is preferably cooled successively by one or more levels of propane refrigeration and by one or more levels of ethane refrigeration and then further cooled by low level methane.

In a typical installation the gas feed at 40 may consist of:

Wt. percent N and CO 5.72 CH 83.60 C H 7.65 C H 2.09 0.; 1

The fuel cut, which was less than 12% of the total gas handled, is effectively removed with the nitrogen and CO by expansion in the stripper 74 as from the initial pressure of 470 p.s.i.g. to about 50 p.s.i.g. In an integrated system such as described, this fuel gas may be used for the operation of steam boilers for steam prime movers or directly in gas turbines as for the centrifugals.

It will be appreciated that the large number of compressors necessarily requires large prime movers. It is estimated for example that the liquefaction of one hundred million standard cubic feet per day of natural gas will require approximately 70,000 horsepower, installed, for compressors and accessories. This is based on the availability of cooling water at an annual average temperature of about 70 F. Such a low temperature is possible in some localities only by submerging the suction pipe 156 as shown in PEG. 1 at least ten and preferably as much as thirty feet below the sea level. This water, used in heat exchanger 128 primarily (and incidentally for the steam condensation), makes it possible to minimize the compression pressure on the three stage compressors 122, 112, and 106 in the propane circuit, and thus reduce the total overall requirements of horsepower. If the normal water temperature were 90 F.,

d for example, an added 10,000 horsepower for pumps would be required.

The invention thus relates to the movable platform on which the gas liquefaction plant can be pre-fabricated, and the platform may be in the form of a seadrome which can be floated across a body of water from an industrial shop to a source of gas, and then elevated out of the water.

This not only makes available immediately a completely integrated liquefaction plant but such a plant can take advantage of the sub-surface water to utilize the highest efiiciency of heat transfer. With the size of plant heretofore described, the water requirements for cooling alone are estimated to exceed 120,000 gallons per minute and to transport such water to a land based unit not only would require pipes in excess of eleven feet in diameter but would involve installation costs more than the cost of the entire marine platform. The saving of such exense materially reduces the investment and maintenance for such a gas liquefaction plant. In addition, the prefabricated gas liquefaction plant and platform can be used as a dock to which liquid carrying tankers can tie up during loading.

The invention thus consists in the novel assembly of a pre-fabricated gas liquefaction plant mounted on a platform, which platform is convertible to a seaworthy vessel and adapted to be towed to a material site whereit is anchored in a suitable manner preferably out of the water as by the use of a caisson and jacks.

Referring again to FIG. 1 some of the apparatus heretofore described is shown in schematic relation. A catalog of such elements follows:

130-Turbine ISL-Ethane compressor 134-Ethane cooler 136Ethane condenser 137Ethane refrigeration drum 138Propane suction drum 140Propane flash drum 142Deethanizer tower 144-Demethanizer tower 145Deethanizer reboiler f48-Demethanizer reboiler Heat exchangers 152-Heat exchangers 1 tCooling water pumps lid-Water inlet The foregoing description of a preferred form of embodiment of the invention will be understood as drawn for the skilled in the art and changes may be made in the details within the scope and spirit of the description herein and the claims appended hereinafter.

We claim:

1. A mobile gas liquefaction plant comprising a multilevel buoyant platform adapted to be transported over water to a relatively deep off-shore site and containing a plurality of marine fioor engaging caissons, means to depress said caissons against the marine floor at the predetermined site to support said platform from said marine floor substantially above the water surface, said platform having means adapted to liquefy a feed gas including a gas liquefaction assembly mounted on multilevels of said platform, said assembly having a gaseous material inlet conduit adapted to be interconnected to a fixed gas supply at said off-shore site, the gas being liquetied in said liquefaction assembly by an external refrigeration circuit comprising a plural stage compressor, a condenser cooled by said water, expansion means and a first evaporator at a first pressure, the said first evaporator being in heat exchange relation with the gas supply to reduce the temperature thereof, a second evaporator connccted to the said refrigeration circuit for expanding refrigerant to a second pressure lower than the first pressure and further cooling the gas supply at a point in the supply line downstream from the first evaporator to liquefy the supply, conduit means returning evaporator refrigerant from the second evaporator to an early stage of compression and evaporated refrigerant from the first evaporator to a last stage of compression.

2. A mobile gas liquefaction plant, as claimed in claim 1, wherein the cooling water inlet to said water cooled condenser is connected to a water source substantially below the Water surface whereby said condenser is cooled by water at a temperature lower than ambient.

3. A mobile gas liquefaction plant, as claimed in claim 2, wherein the cooling water inlet connection extends at least ten feet below the water surface.

4. A mobile gas liquefaction plant, as claimed in claim 1, in which the external refrigeration circuit includes at 15 least three separate refrigerant systems which use refrigerants from the class of propane, ethane, methane and nitrogen.

5. A mobile gas liquefaction plant as claimed in claim 4, in which the separate refrigerant systems incrementally cool the temperature of the gas to about 258 F.

References Cited in the tile of this patent UNITED STATES PATENTS 2,198,098 Vaughan Apr. 29, 1940 2,224,227 Keith, et al Dec. 10, 1940 2,509,034 Claitor et al. May 23, 1950 2,627,318 Swerdlofi" Feb. 3, 1953 2,775,869 Pointer Jan. 1, 1957 2,795,937 Sattler et a1 June 18, 1957 2,896,414 Tung July 28, 1959 2,927,436 Besse Mar. 8, 1960 2,940,268 Morrison June 14, 1960 2,959,020 Knapp Nov. 8, 1960 2,960,837 Swenson et al. Nov. 22, 1960 2,973,834 Cicalese Mar. 7, 1961 3,020,723 De Lury et a1 Feb. 13, 1962 

1. A MOBILE GAS LIQUEFACTION PLANT COMPRISING A MULTILEVEL BUOYANT PLATFORM ADAPTED TO BE TRANSPORTED OVER WATER TO A RELATIVELY DEEP OFF-SHORE SITE AND CONTAINING A PLURALITY OF MARINE FLOOR ENGAGING CAISSONS, MEANS TO DEPRESS SAID CAISSONS AGAINST THE MARINE FLOOR AT THE PREDETERMINED SITE TO SUPPORT SAID PLATFORM FROM SAID MARINE FLOOR SUBSTANTIALLY ABOVE THE WATER SURFACE, SAID PLATFORM HAVING MEANS ADAPTED TO LIQUEFY A FEED GAS INCLUDING A GAS LIQUEFACTION ASSEMBLY MOUNTNED ON MULTILEVELS OF SAID PLATFORM, SAID ASSEMBLY HAVING A GASEOUS MATERIAL INLET CONDUIT ADAPTED TO BE INTERCONNECTED TO A FIXED GAS SUPPLY AT SAID OFF-SHORE SITE, THE GAS BEING LIQUEFIED IN SAID LIQUEFACTION ASSEMBLY BY AN EXTERNAL REFRIGERATION CIRCUIT COMPRISING A PLURAL STAGE COMPRESSOR, A CONDENSER COOLED BY SAID WATER, EXPANSION MEANS AND A FIRST EVAPORATOR AT A FIRST PRESSURE, THE SAID FIRST EVAPORATOR BEING IN HEAT EXCHANGE RELATION WITH THE GAS SUPPLY TO REDUCE THE TEMPERATURE THEREOF, A SECOND EVAPORATOR CONNECTED TO THE SAID REFRIGERATION CIRCUIT FOR EXPANDING REFRIGERANT TO A SECOND PRESSURE LOWER THAN THE FIRST PRESSURE AND FURTHER COOLING THE GAS SUPPLY AT A POINT IN THE SUPPLY LINE DOWNSTREAM FROM THE FIRST EVAPORATOR TO LIQUEFY THE SUPPLY, CONDUIT MEANS RETURNING EVAPORATOR REFRIGERANT FROM THE SECOND EVAPORATOR TO AN EARLY STAGE OF COMPRESSION AND EVAPORATED REFRIGERANT FROM THE FIRST EVAPORATOR TO A LAST STAGE OF COMPRESSION. 